IEEE ELECTRON DEVICE LETTERS, VOL. 37, NO. 3, MARCH 2016 345

High-Current Tunneling FETs With (11̄0)Orientation and a Channel Heterojunction

Pengyu Long, Jun Z. Huang, Michael Povolotskyi, Gerhard Klimeck, Fellow, IEEE,and Mark J. W. Rodwell, Fellow, IEEE

Abstract— We propose InAs/GaSb ultrathin-body tunnelingfield-effect transistors (TFETs) using confinement in the (11̄0)plane and transport in the [110] direction to increase thetunneling probability by reducing the tunnel barrier energy andhole effective mass. To reduce the OFF-state leakage current, weadd an InAs/In1−nAlnAs1−nSbn heterojunction to the channel,which increases the valence band barrier. The heterojunction alsoincreases the tunneling probability and ON-current by reducingthe tunneling distance through the p-n junction and introducing aresonant state. A fully atomistic non-equilibrium Green functionquantum transport approach in NEMO5 is used to explore thedesign space. While choosing 10−3 A/m OFF-current (IOFF) anda 0.3 V power supply, we simulate 270 A/m ON-current (ION)for a 30-nm gate length and 170 A/m for a 15-nm gatelength (L g), while a conventional 15-nm L g GaSb/InAs TFETunder (001) confinement shows only 24 A/m ION.

Index Terms— Tunnel FETs, crystal orientation, bandstructure engineering, resonant tunneling.

I. INTRODUCTION

FUTURE VLSI performance is constrained by power dis-sipation [1]. Low switching power calls for a low supply

voltage, yet decreased standby power calls for either increasedvoltages or reduced transistor subthreshold swing (S.S.).In conventional MOSFETs, the S.S. is limited to 60mV/decby thermal injection [1]. Though tunnel FETs (TFETs) [2]–[4]can obtain smaller S.S., their ON current (ION) is limitedby the PN junction tunneling probability [3], which is deter-mined by the carrier effective mass and tunneling distance.In nanoscale TFETs confinement quantizes subbands and

Manuscript received January 11, 2016; revised January 26, 2016;accepted January 27, 2016. Date of publication January 28, 2016; dateof current version February 23, 2016. This work uses nanoHUB.orgcomputational resources operated by the Network for ComputationalNanotechnology funded by the U.S. National Science Foundationunder Grant EEC-0228390, Grant EEC-1227110, Grant EEC-0634750,Grant OCI-0438246, Grant OCI-0832623, and Grant OCI-0721680. Thismaterial is based upon work supported by the National Science Foundationunder Grant 1125017. NEMO5 developments were critically supported by anNSF Peta-Apps award OCI-0749140 and by Intel Corp. The review of thisletter was arranged by Editor S. J. Koester.

P. Long, J. Z. Huang, M. Povolotskyi, and G. Klimeck are withthe Network for Computational Nanotechnology and Birck Nanotechnol-ogy Center, Purdue University, West Lafayette, IN 47907 USA (e-mail:davidlong180@gmail.com).

M. J. W. Rodwell is with the Department of Electrical and ComputerEngineering, University of California at Santa Barbara, Santa Barbara,CA 93106-9560 USA.

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2016.2523269

therefore increases the barrier energy and non-parabolic dis-persions increase the carrier effective masses; both effectsresult in a decreased tunneling probability and reduced ION.The use of InAs/GaSb PN tunnel barriers reduces the tunnelingbarrier energy and tunneling distance, however at the desiredlayer thicknesses of a few nm, strong confinement and non-parabolic effects still reduce ION significantly. Alternativemethods are needed to further increase ION.

Here we propose the concept to increase the confinementeffective mass (to lower the confinement energy) and todecrease the transport effective mass (to increase the tun-neling current). This can be achieved in the InAs mate-rial system by choosing the (11̄0) plane for confinementand the [110] direction for transport. We also introduce anInAs/In1−nAlnAs1−nSbn heterojunction into the channel, andtherefore resonantly enhance the tunneling probability in theON-state and reduce the OFF-state source-drain tunnelingleakage current.

II. CONFINEMENT AND TRANSPORT DIRECTIONS

III-V materials have strongly anisotropic heavy-holebands [5], [6] hence 2-D hole subband structureis a strong function [5]–[7] of the confinement plane.(11̄0) confinement provides subbands with low effectivemass [8], particularly given [110] transport [9], [10]. Fora tch = 2 nm thick channel, the hole effective massreduces from 0.131m0 given (001) confinement and [100]transport, or 0.19m0 given [110] transport, to 0.073m0given (11̄0) confinement and [110] transport. Further,because the heavy hole mass is larger in <110> thanin <100>, the heavy hole subband energy shifts more slowlyunder (11̄0) confinement than under (100). Compared to (100)confinement, at 2 nm tch , (11̄0)-confined TFETs have lowerhole effective mass and a smaller tunnel barrier energy(0.11eV vs. 0.15eV) between the GaSb source valence bandand the InAs channel conduction band (Fig. 1a, b). The tunnelbarrier is consequently thinner (Fig. 1c) and the tunnelingprobability higher (Fig. 1d).

The designs are based on a p-GaSb/n-InAs double-gateTFET (Fig. 2a). The gate oxide thickness is labeled tox anddielectric constant εr . The gate length is Lg , and source anddrain doping densities are labeled NS and ND . The channelwidth extends in the z direction (perpendicular to the page).Table I lists the various design parameters. For (001)-confined

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346 IEEE ELECTRON DEVICE LETTERS, VOL. 37, NO. 3, MARCH 2016

Fig. 1. Conduction band structure of InAs and valence band structure ofGaSb for a) (11̄0) confined, and b) (001) confined UTBs. c) Band diagramunder ON-state bias and d) transmission probability of (11̄0) confined and(001) confined UTB GaSb/InAs TFETs. The channels are 2 nm thick, andLg = 15 nm.

Fig. 2. Cross-section of a) a conventional GaSb/InAs TFET. b) Newlydesign heterostructure GaSb/InAs TFET with an InAs/ In1−n AlnAs1−nSbnheterojunction in the channel.

TFETs, the transport direction is [100], and the GaSb/InAsheterointerface is InSb-like. For (11̄0)-confined TFETs, thetransport direction is [110]. The channel is 2 nm thick; at15 nm Lg , thicker channels suffer large source-drain tunnelingleakage current. III-V MOSFETs with Tch = 2.5 nm have beenreported [11], [12].

Devices are simulated using the atomistic nanoelectronicmodeling tool NEMO5 [13], which solves self-consistently thePoisson equation and the open boundary Schrödinger equation(quantum transmitting boundary method [14], [15]), using the300K tight binding parameters of [16] and [17]; these give,in bulk, a 0.197 eV offset between the GaSb valence band andthe InAs conduction band [18]. The InAlAsSb parameters arelinearly interpolated using the virtual crystal approximationand are benchmarked against [19]. We integrate over the1D Brillouin zone using a high order Gauss quadrature with20 points. The simulations do not model carrier scattering.In [20]–[22], it is shown that phonon scattering slightlyreduces TFET Ion , but does not significantly increase Io f f .In the on-state, if carrier scattering is strong, the degree of res-onant enhancement of the transmission will be decreased, andwill degrade Ion . Trap-assisted tunneling and Shockley-Read-Hall thermal generation may also increase Io f f and degradeS.S. [23]–[26] Increasing the alloy fraction n within theIn1−nAlnAs1−nSbn channel will increase the channel bandgapand hence reduce thermal generation leakage currents.

Figure 1d demonstrates that in the energy range betweenthe GaSb valence band and InAs conduction band, the TFEThas a higher transmission probability with (11̄0) comparedto (001) confinement. At 30 nm Lg , ION increases from

TABLE I

PARAMETERS FOR GaSb/InAs TUNNELING FETs

Fig. 3. Off-state transmission probability a) and band diagram b) of(11̄0)-confined GaSb/InAs TFETs with and without an InAs/In1−n AlnAs1−nSbn channel heterojunction. The transmission probability c) and banddiagram d) are also shown in the ON state. x is the alloy fraction.

30A/m to 80A/m (VDD = 0.3V, IOFF = 10−3A/m) comparingto a (001) confined TFET. However, undesired source-draintransmission at energies below the InAs conduction band edgeis increased in the (11̄0)-confined case (Fig. 1d) because InAsthen has a smaller quantized bandgap (Fig. 1c) and smallerhole effective mass. The two-band S/D tunneling probabilityincreases, and, at 15 nm Lg , the S.S. is degraded and, at fixedIOFF, ION reduced.

III. CHANNEL HETEROJUNCTION

The introduction of an InAs/In1−xAlx As1−xSbx channelheterojunction (Fig. 2b) can improve both ON- and OFF-state performance, as reported in [27] and [28]. In the OFF-state, the heterojunction increases the valence-band barrier atthe channel-source interface. This reduces the (two-band) S/Dtunneling probability (Fig. 3a) in part because, under off-statebias (Fig. 3b), the energy separation between the InAs valenceband and S/D tunneling evanescent states is increased, and inpart because In1−nAlnAs1−nSbn has slightly larger electroneffective mass (0.067m0) than InAs (0.052m0). The width ofthe InAs well is adjusted so that the single resonant state isaligned with InAlAsSb conduction band; in the InAs layer,there are no states at energies below the InAlAsSb conductionband. The lower edge of the transmission characteristics(Fig. 3a) becomes sharper, resulting in a steeper S.S., as willbe seen subsequently. InAs is retained at the PN junction tomaintain a small tunnel barrier energy for high ION.

Under ON-state bias (Fig. 3d), i.e. with the channel conduc-tion band ∼0.2eV below the source valence band, inserting the

LONG et al.: HIGH-CURRENT TUNNELING FETs WITH (11̄0) ORIENTATION AND A CHANNEL HETEROJUNCTION 347

Fig. 4. Local density of states in the a) OFF state and b) ON state ofa GaSb/InAs TFET with an InAs/In1−n AlnAs1−nSbn (n = 0.14) channelheterojunction. The circled area indicates the resonant state.

Fig. 5. a) Transmission probability and b) band diagram of the GaSb/InAstunneling junction for different InAs/In1−n AlnAs1−nSbn heterojunctions.In all designs, the resonant state energy lies immediately above theIn1−n AlnAs1−nSbn conduction band. The TFETs have (11̄0) confinement.

channel heterojunction increases the field in the PN tunnelbarrier, reducing the PN junction tunneling distance henceincreasing the tunneling probability (Fig. 3c). Inserting thechannel heterojunction also introduces a resonant state inthe InAs layer between the PN junction and the channelheterojunction (Fig. 4b). The tunneling probability is furtherincreased by adjusting the In1−nAlnAs1−nSbn alloy fraction toalign the resonant state energy to the channel conduction-bandenergy (Fig. 3c). This further improves ION.

To investigate the effect of the channel heterojunction onION, independent of its effect on S/D tunneling, we firstconsider the case of 30 nm Lg . We simulate (Fig. 5b) a seriesof designs with varying InAs channel length d , while varyingthe In1−nAlnAs1−nSbn alloy fraction x such that the lowestresonant state in the InAs layer always lies immediately abovethe In1−nAlnAs1−nSbn conduction band.

As the InAs layer is progressively thinned and the alloy frac-tion n simultaneously increased, the InAs/In1−nAlnAs1−nSbn

conduction band offset increases, and the energy separationbetween the InAs resonant state (aligned with theIn1−nAlnAs1−nSbn conduction band) and the InAsconduction band increases (Fig. 5(b)). The PN junctiontunneling distance therefore progressively decreases (Fig. 5b),progressively increasing ION (Fig. 6a). With VDD = 0.3V and

Fig. 6. Transfer characteristics of a) 30 nm Lg and b) 15 nm Lg(11̄0)-confined TFETs for different In1−nAlnAs1−nSbn heterojunction bandprofiles. Results are compared to (11̄0)- and (001)-confined TFETs with nochannel heterojunction.

IOFF = 10−3A/m, and with (11̄0) confinement, ION increasesfrom 80A/m (n=0) to 270A/m (n = 0.14, d = 4.5 nm).

At 15 nm Lg , S/D tunneling is larger, and theInAs/In1−nAlnAs1−nSbn heterojunction provides a greaterimprovement in the ON/OFF current ratio. With VDD = 0.3Vand IOFF = 10−3A/m, and with (11̄0) confinement, as theAlSb mole fraction n increases, ION increases from5A/m (n = 0) to 170A/m (n = 0.14, d = 4.5 nm).The (001)-confined TFET shows 24A/m ION.

A similar P-channel TFET design would use an N-InAssource, a channel containing a GaSb/AlGaSb heterojunction,a P+ AlGaSb drain, (11̄0) confinement, and [110] transport.

IV. SUMMARY

It is found that using the (11̄0) confinement plane andthe [110] transport direction improves ION comparedto (001)- confined TFETs. Introducing an InAs/In1−nAlnAs1−nSbn channel heterojunction reduces theS/D tunneling and improves the S.S. Further, the channelheterojunction greatly increases ION, both by decreasing thetunneling distance and by introducing a resonant state. Withinthis design space a 15 nm gate length TFET with 170A/mON current can be obtained.

REFERENCES

[1] T. N. Theis and P. M. Solomon, “In quest of the ‘next switch’: Prospectsfor greatly reduced power dissipation in a successor to the siliconfield-effect transistor,” Proc. IEEE, vol. 98, no. 12, pp. 2005–2014,Dec. 2010. DOI: 10.1109/JPROC.2010.2066531

[2] Q. Zhang, W. Zhao, and A. Seabaugh, “Low-subthreshold-swing tunneltransistors,” IEEE Electron Device Lett., vol. 27, no. 4, pp. 297–300,Apr. 2006. DOI: 10.1109/LED.2006.871855

[3] S. O. Koswatta, M. S. Lundstrom, and D. E. Nikonov, “Perfor-mance comparison between p-i-n tunneling transistors and conventionalMOSFETs,” IEEE Trans. Electron Devices, vol. 56, no. 3, pp. 456–465,Mar. 2009. DOI: 10.1109/TED.2008.2011934

[4] G. Dewey, B. Chu-Kung, J. Boardman, J. M. Fastenau, J. Kavalieros,R. Kotlyar, W. K. Liu, D. Lubyshev, M. Metz, N. Mukherjee,P. Oakey, R. Pillarisetty, M. Radosavljevic, H. W. Then, andR. Chau, “Fabrication, characterization, and physics of III–V heterojunc-tion tunneling field effect transistors (H-TFET) for steep sub-thresholdswing,” in Proc. IEEE Int. Electron Devices Meeting, Dec. 2011,pp. 33.6.1–33.6.4. DOI: 10.1109/IEDM.2011.6131666

[5] G. Klimeck, R. C. Bowen, and T. B. Boykin, “Strong wavevector depen-dence of hole transport in heterostructures,” Superlattices Microstruct.,vol. 29, no. 3, pp. 187–216, Mar. 2001. DOI: 10.1006/spmi.2000.0973

348 IEEE ELECTRON DEVICE LETTERS, VOL. 37, NO. 3, MARCH 2016

[6] N. Neophyto, A. Paul, and G. Klimeck, “Bandstructure effects in siliconnanowire hole transport,” IEEE Trans. Nanotechnol., vol. 7, no. 6,pp. 710–719, Nov. 2008. DOI: 10.1109/TNANO.2008.2006272

[7] J. Z. Huang, L. Zhang, P. Long, M. Povolotskyi, and G. Klimeck. (2015).“Quantum transport simulation of III–V TFETs with reduced-orderK.P method.” [Online]. Available: http://arxiv.org/abs/1511.02516

[8] N. Neophytou and G. Klimeck, “Design space for low sensitivity tosize variations in [110] PMOS nanowire devices: The implicationsof anisotropy in the quantization mass,” Nano Lett., vol. 9, no. 2,pp. 623–630, 2009. DOI: 10.1021/nl802893m

[9] J. Wang, P. M. Solomon, and M. Lundstrom, “A general approachfor the performance assessment of nanoscale silicon FETs,”IEEE Trans. Electron Devices, vol. 51, no. 9, pp. 1366–1370.DOI: 10.1109/TED.2004.833962

[10] N. Neophytou, G. Klimeck, and H. Kosina, “Subband engineeringfor p-type silicon ultra-thin layers for increased carrier velocities:An atomistic analysis,” J. Appl. Phys., vol. 109, no. 5, p. 053721,2011. DOI: 10.1063/1.3556435

[11] S. Lee, V. Chobpattana, C.-Y. Huang, B. J. Thibeault, W. Mitchell,S. Stemmer, A. C. Gossard, and M. J. W. Rodwell, “Record Ion(0.50 mA/μm at VDD = 0.5 V and Ioff = 100 nA/μm) 25 nm-gate-length ZrO2/InAs/InAlAs MOSFETs,” in IEEE Symp. VLSI Technol.,Dig. Tech. Papers, Jun. 2014, pp. 1–2.

[12] C.-Y. Huang, P. Choudhary, S. Lee, S. Kraemer, V. Chobpattana,B. Thibeault, W. Mitchell, S. Stemmer, A. Gossard, and M. Rodwell,“12 nm-gate-length ultrathin-body InGaAs/InAs MOSFETs with8.3 · 105 ION/IOFF,” in Proc. 73rd Annu. Device Res. Conf. (DRC),Jun. 2015, p. 260. DOI: 10.1109/DRC.2015.7175669

[13] J. E. Fonseca, T. Kubis, M. Povolotskyi, B. Novakovic, A. Ajoy,G. Hegde, H. Ilatikhameneh, Z. Jiang, P. Sengupta, Y. Tan, andG. Klimeck, “Efficient and realistic device modeling from atomic detailto the nanoscale,” J. Comput. Electron., vol. 12, no. 4, pp. 592–600,2013. DOI: 10.1007/s10825-013-0509-0

[14] C. S. Lent and D. J. Kirkner, “The quantum transmitting bound-ary method,” J. Appl. Phys., vol. 67, no. 10, pp. 6353–6359, 1990.DOI: 10.1063/1.345156

[15] M. Luisier, A. Schenk, W. Fichtner, and G. Klimeck, “Atomisticsimulation of nanowires in the sp3d5s∗ tight-binding formalism: Fromboundary conditions to strain calculations,” Phys. Rev. B, vol. 74, no. 20,p. 205323. Nov. 2006. DOI: 10.1103/PhysRevB.74.205323

[16] Y. P. Tan, M. Povolotskyi, T. Kubis, T. B. Boykin, and G. Klimeck,“Tight-binding analysis of Si and GaAs ultrathin bodies with subatomicwave-function resolution,” Phys. Rev. B, vol. 92, no. 8, p. 085301, 2015.DOI: 10.1103/PhysRevB.92.085301

[17] Y. Tan et al. Tight Binding Parameters by DFT Mapping.https://nanohub.org/resources/15173, accessed Jan. 26, 2016.

[18] I. Vurgaftman, J. R. Meyer, and L. R. Ram-Mohan, “Band parametersfor III–V compound semiconductors and their alloys,” J. Appl. Phys.,vol. 89, no. 11, pp. 5815–5875, 2001. DOI: 10.1063/1.1368156

[19] T. H. Glisson, J. R. Hauser, M. A. Littlejohn, C. K. Williams,“Energy bandgap and lattice constant contours of III–V quater-nary alloys,” J. Electron. Mater., vol. 7, no. 1, pp. 1–16, 1978.DOI: 10.1007/BF02656016

[20] M. Luisier and G. Klimeck, “Simulation of nanowire tunneling transis-tors: From the Wentzel–Kramers–Brillouin approximation to full-bandphonon-assisted tunneling,” J. Appl. Phys., vol. 107, no. 8, p. 084507,2010. DOI: 10.1063/1.3386521

[21] S. O. Koswatta, S. J. Koester, and W. Haensch, “On the possibility ofobtaining MOSFET-like performance and sub-60-mV/dec swing in 1-Dbroken-gap tunnel transistors,” IEEE Trans. Electron Devices, vol. 57,no. 12, pp. 3222–3230, Dec. 2010. DOI: 10.1109/TED.2010.2079250

[22] U. E. Avci, D. H. Morris, S. Hasan, R. Kotlyar, R. R. Kim, R. Rios,D. E. Nikonov, and I. A. Young, “Energy efficiency comparisonof nanowire heterojunction TFET and Si MOSFET at Lg=13 nm,including P-TFET and variation considerations,” in Proc. IEEEInt. Electron Devices Meeting, Dec. 2013, pp. 33.4.1–33.4.4.DOI: 10.1109/IEDM.2013.6724744

[23] U. E. Avci, B. Chu-Kung, A. Agrawal, G. Dewey, V. Le, R. Rios,D. H. Morris, S. Hasan, R. Kotlyar, J. Kavalieros, and I. A. Young,“Study of TFET non-ideality effects for determination of geometry anddefect density requirements for sub-60 mV/dec Ge TFET,” in Proc.IEEE Int. Electron Devices Meeting (IEDM), 2015, pp. 34.5.1–34.5.4.

[24] M. G. Pala and D. Esseni, “Interface traps in InAs nanowire tunnel-FETs and MOSFETs—Part I: Model description and single trapanalysis in tunnel-FETs,” IEEE Trans. Electron Devices, vol. 60, no. 9,pp. 2795–2801, Sep. 2013 DOI: 10.1109/TED.2013.2274196

[25] D. Esseni and M. G. Pala, “IInterface traps in InAs nanowire tunnelFETs and MOSFETs—Part II: Comparative analysis and trap-inducedvariability,” IEEE Trans. Electron Devices, vol. 60, no. 9,pp. 2802–2807, Sep. 2013. DOI: 10.1109/TED.2013.2274197

[26] S. Mookerjea, D. Mohata, T. Mayer, V. Narayanan, and S. Datta,“Temperature-dependent I–V characteristics of a verticalIn0.53Ga0.47As tunnel FET,” IEEE Electron Device Lett., vol. 31,no. 6, pp. 564–566, Jun. 2010. DOI: 10.1109/LED.2010.2045631

[27] K. Ganapathi and S. Salahuddin, “Heterojunction vertical band-to-bandtunneling transistors for steep subthreshold swing and high on current,”IEEE Electron Device Lett., vol. 32, no. 5, pp. 689–691, May 2011.DOI: 10.1109/LED.2011.2112753

[28] M. G. Pala and S. Brocard, “Exploiting hetero-junctions toimprove the performance of III–V nanowire tunnel-FETs,” IEEEJ. Electron Devices Soc., vol. 3, no. 3, pp. 115–121, May 2015,DOI: 10.1109/JEDS.2015.2395719